The present invention is directed toward a wavelength division multiplexed communication system having a dispersion compensation element that provides spectrally uniform nonlinearity behavior.
Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information. Although the pulses are usually at a single nominal wavelength, each pulse is actually composed different spectral components. These spectral components propagate through the transmission fiber at different speeds with higher frequency components traveling slower than lower frequency components. This effect, known as xe2x80x9cchromatic dispersionxe2x80x9d, can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity. Chromatic dispersion becomes increasingly pronounced at higher bit rates, e.g. those associated with synchronous optical network (SONET) OC-192 transmission speeds.
Dispersion compensated fiber, commercially available from Corning, for example, can be used to offset chromatic dispersion. It is known, however, that dispersion compensated fiber has a nonlinearity coefficient xcex3, which is related to a nonlinearity property of the refractive index of the fiber, n2, and the mode field diameter, otherwise referred to as the fiber effective area Aeff (see Agrawal, xe2x80x9cNonlinear Fiber Opticsxe2x80x9d, Academic Press, Inc., 1995, pp. 37-43). In particular, xcex3 can be expressed as follows:   γ  =            2      ⁢      π      ⁢              xe2x80x83            ⁢              n        2                    λ      ⁢              xe2x80x83            ⁢              A        eff            
In general, n2 depends on the fiber composition and dopants, e.g., fluoride doped fibers have a different n2 value than germanium doped fibers. Aeff, however, is related to fiber geometry, doping profile and waveguiding characteristics. For single wavelength transmission applications, n2 and Aeff have fixed values associated with the transmission wavelength xcex.
Recently, wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. The wavelengths are typically within a narrow range about 1550 nm, the absorption minimum of silica fiber. At high data speeds, such as OC-192 rates, the dispersion associated with each channel must be compensated. Moreover, the nonlinearity coefficient xcex3 for each channel should be substantially the same, i.e., xcex3 should be spectrally uniform. Otherwise, some channels may have more errors than others, thereby degrading system performance.
The nonlinearity coefficient xcex3 of dispersion compensated fiber has been assumed to be spectrally uniform over a relatively broad range of optical wavelengths. It has been found, however, that xcex3 can vary substantially over a relatively narrow wavelength range of wavelengths. In particular, for wavelengths within a narrow range about 1550 nm, n2/Aeff values of dispersion compensated fiber as high as 1.7xc3x9710xe2x88x929 (1/W) (at about 1543 nm) and as low as low as 1.2xc3x9710xe2x88x929 (1/W) (at about 1557 nm) have been measured. Accordingly, in WDM systems in which optical signals carry high-speed data at these wavelengths, selected channels can have a 40% higher xcex3 than other channels. As a result, the higher xcex3 channels may exhibit different system performance as characterized by different error rates. Accordingly, system design is made difficult.
Accordingly, the present invention provides a WDM optical communication apparatus comprising a dispersion compensating element having at least one dispersion compensating fiber. The dispersion compensating element is configured to mitigate the effects of a spectrally nonuniform nonlinearity coefficient behavior of the fiber, thereby giving an effectively uniform nonlinearity behavior for the plural optical signals in a WDM system.